Endogenous opioid peptides, first reported by Hughes and coworkers in 1975 (Hughes et al., Nature 258: 577-580, 1975), have been documented to be potent regulators of growth (Zagon and McLaughlin, Opioid growth factor in the developing nervous system, in: I. S. Zagon and P. J. McLaughlin (Eds.), Receptors in the Developing Nervous System, vol. 1, Growth Factors and Hormones, Chapman and Hall, London, UK, 1993, pp. 39-62), as well as neuromodulators (Akil et al., Ann. Rev. Neurosci. 7: 223-255, 1984). One native opioid peptide, [Met5]-enkephalin, has been reported to be an inhibitory growth factor in development, cellular renewal, cancer, wound healing, and angiogenesis (Isayama, et al., Brain Res. 544: 79-85, 1991; McLaughlin, Amer. J. Physiol. 271: R122-129, 1996; Murgo, J. Natl. Cancer Inst. 75: 341-344, 1995; Steine-Martin, et al., Life Sci. 46: 91-98, 1990; Villiger et al., EMBO J 11: 135-143, 1992; Zagon and McLaughlin, 1993, supra; Zagon et al., Amer. J. Physiol 271: R780-R786, 1996; Zagon et al., Brain Res. 798: 254-260, 1998; Zagon et al., Brain Res. 803: 61-68, 1998. In view of these growth properties, [Met5]-enkephalin has been termed opioid growth factor (OGF) (Zagon and McLaughlin, 1993, supra). OGF is an autocrine produced and secreted peptide that is not cell, tissue, or organ specific. While OGF exhibits activity at physiologically relevant concentrations, it does not elicit physical dependence, tolerance, and/or withdrawal. OGF displays a temporal and spatial distribution consistent with specific growth-related effects and is sensitive to opioid antagonist displacement. OGF has a direct, rapid, prolonged, stereospecific, receptor mediated, non-cytotoxic, and reversible influence on growth both in tissue culture and in prokaryotic and eukaryotic organisms. Blockade of the interaction between endogenous opioids and opioid receptors with compounds such as naltrexone (NTX) enhances growth (McLaughlin et al., Physiol. Behav. 62: 501-508, 1997; Zagon, et al., Science 221: 671-673, 1983; Zagon et al., Science 221: 1179-1180, 1983), suggesting that growth related opioid peptides such as OGF are tonically active. The molecular nature of OGF is well documented, and this peptide is encoded by the preproenkephalin gene (Gubler et al., Nature 295: 206-209, 1982; Noda et al. Nature 295: 202-206, 1982).
The receptor mediating the action of OGF shares certain pharmacological characteristics of classical opioid receptors, including the binding to opioids, stereospecificity, and naloxone-reversibility (Zagon et al., Brain Res. 551: 28-35, 1991; Zagon et al., Brain Res. 482: 297-305, 1989). Thus, this receptor was originally—and tentatively—termed the zeta (ζ) opioid receptor. However, physiological, pharmacological, receptor binding assays and immunocytochemical localization experiments have revealed the novel nature of this receptor. In particular, the function (growth), tissue distribution (neural and non-neural), subcellular location (nuclear-associated), transient appearance during ontogeny, ligand specificity ([Met5]-enkephalin), and competitive inhibition profile differ substantially from what is known about classical opioid receptors.
The present invention provides for the first time the molecular information of the receptor for OGF, in particular, the nucleotide and amino acid sequences of such receptor. Comparison of such sequences with those reported for the opioid receptor family shows no structural homology. In view of the pharmacological, biochemical, physiological and molecular differences, the present invention has termed the receptor tentatively identified as the zeta opioid receptor, the OGF receptor (OGFr).